Method and apparatus for the separation of solid particles into sized fractions



Jan. 19, 1965 D. F. KELSALL 3,166,496 METHOD AND APPARATUS FORTHE SEPARATION OF soun PARTICLES INTO SIZED FRACTIONS Filed July 25 1961 2 Sheets-Sheet 1 ON 0F SOLID PARTICLES INTO SIZED FRACTIONS Filed July 25. 1961 Jan. 19, 1965 B. F. KELSALL METHOD AND APPARATUS FOR THE SEPARATI 2 Sheets-Sheet 2 United States Patent ()fifice 3,166,496 METHOD AND APPARATUS FOR THE SEPARA- TIQN OF SOLID PARTICLES INTO IZED FRACTIONS Denis Fletcher Kelsall, Sandringham, Victoria, Australia, assignor to Commonwealth Scientific and Industrial Research Organization, East Melbourne, Victoria, Australia, a body corporate Filed July 25, 1961, Ser. No. 126,612 Claims priority, application Australia, Aug. 1, 1960, 63,071/60 4 Claims. (Cl. 209-211) This invention relates to a method and apparatus for the separation of solid particles into sized fractions by a process of elutriation. It is frequently desirable as for example in the mining industry to test samples to determine the amount of'solid particles of a particular size. Frequently it is desirable to carry out such a test to separate out particles which are of a size less than the maximum size passed by the smallest conveniently available screens (less than about 40 microns).

There are many designs of apparatus utilizing elutriation for the separation of solid particles into sized fractions or to provide data from which a size analysis can be obtained. A considerable number of these devices are based on principles which utilize differences in rates of settling under the gravitational force in either liquids or gases of selected specific gravity, i.e. they rely on the fact that particles can be separated according to the rate at which they settle through a fluid medium.

, Among the limitations of such types of apparatus (not necessarily applicable to all) may be listed the following.

(a) Some do not produce separate sized fractions (e.g. sedimentation balances and pipette methods).

(11) Someare limited to small samples of the solid particles-less than 1 gm. (sedimentation balances). This, in some cases, makes it exceedingly difficult to obtain representative samples from large amounts of solids.

Some involve complicated and expensive equipment.

' (d) Operation of the apparatus requires training and experience.

(e) There are often errors due to starting conditions.

(1) For accurate results, especially with very fine solids, settling under gravity involves long time intervals and very carefully controlled conditions of temperature (i.e. no draughts which might cause slight eddy currents in the fluid environment).

(g) With those elutriators which produce separate sized fractions the precision of separation is generally poor, each fraction containing considerable amounts of particles both larger and smaller than the ideal limits of the size range.

The principal object of this invention is to provide a simple apparatus which is easy to operate and which eliminates or substantially reduces the disadvantages of, the prior apparatus. To these ends there is employed according to this invention a cyclone elutriator to separate particles according to thefrate at which they settle through a fluid medium'under the influence of centrifugal forces. The settling rate of a particlein a given fluid and under the influence of a given force is dependent upon the size, shape and specific gravity ofthe particle. For ease of description the separation is herein referred to as a separation according to size but it is to be appreciated that the expression size is to be taken to include the abovementioned other factors where these are relevant. In 'short, therefore, the expresison size is used as a convenient term for settling'rate.

According to this invention solid particles are separated into sized fractions by the introduction of the particles into a fluid stream of a cyclone elutriator, the elutriator being modified in that, during elutriation there is no Patented Jan. 19, 1965 net underflow discharge and all discharge of fluid from the system occurs through the vortex finder. This modification ensures that all but the finest particles are repeatedly passed through the cyclone and by suitable selection of the essential dimensions and operating conditions of the cyclone the size fraction discharged through the vortex finder can be accurately delineated. The invention also includes the method of separating solid particles into sized fractions comprising the steps of introducing the particles into the fluid stream of a cyclone elutriator, retaining in the elutriator particles of size greater than a predetermined minimum during a time sufiicient to ensure that they repeatedly traverse the flow pattern of the elutriator and then discharging the said retained particles from the elutriator. Preferably the rate of flow of fluid through the cyclone is considerably greater at starting and just before stopping than it is during the elutriation.

The invention further includes apparatus for use in the above operations.

In order that the invention may be more readily understood there will now be described by way of example and with reference to the accompanying drawings one present practical embodiment of the invention employing a generally conical cyclone elutriator mounted with the apex orifice directed upwards and the vortex finder discharging downwards. Hereafter, for convenience of description, when referring to such an elutriator the underflow discharge will be referred to as the apex discharge and the overflow discharge will be referred to as the vortex finder discharge.

In the drawings:

FIGURE 1 is a graph showing the distribution of sample discharge from a conventional cyclone elutriator, curve X, and from one which is modified in the manner of this invention, curve Y,

FIGURE 2 is a schematic cross-sectional view of one form of cyclone elutriator constructed in accordance with this invention,

FIGURE 3 is a schematic view of an arrangement for operating a number of elutriators in series, and

FIGURE 4 is a schematic view of an alternative method of series of operation of a number of elutriators.

Referring to FIGURE 2, the cyclone 1 comprises an upper conical part 2 and a lower cylindrical part 3. The apex 4 of conical portion is uppermost and this, in a conventional cyclone, would constitute the underflow or apex discharge orifice. The lower cylindrical part 3 is pro vided with a tangential fluid inlet 5 and an overflow dis:- charge orifice or vortex finder 6. Located above the apex 4- of the cyclone and detachable from it is a transparent sided enclosed chamber 7 which acts as a container for the sample under test. When thesample container 7 is in position the apex orifice 4 communicates the interior of the cyclone with the interior of the samplecontainer so that the sample container in effect encloses the apex orifice.

The sample container 7 carries a metal rod 8 which has a rubber plug 9 attached to it. This rubber plug is used initially to seal off the opening 10 in the bottom of the sample container as will be later explained. The sample containeralso carries a small discharge valve Ill for discharging the contents of the elutriator at the end of the test.

In order to explain the principles involved in this invention it is necessary to consider the operation of a conventional hydraulic cyclone elutriator of identical dimensions but with no sample container attached to the apex orifice. Under these circumstances the solid particles would be introduced with the fluid through the tangential feed opening. Due to the well known cyclone action the faster settling particles would be discharged with a portion of the fluid through the apex orifice 4 and the remaining, or slower settling particles, discharged with the remaining fluid through the vortex finder or overthrow orifice 6. The effective size of separation would be dependent on the well known variables e.g. flow rate of fluid, tangential feed inlet conduit diameter, vortex finder length and diameter and apex orifice diameter, etc. In a separation using such a cyclone all particles above a certain size would be discharged through the apex opening but the remaining particles would be discharged partly through the apex opening and partly through the vortex finder so that for particles below that certain size it is not possible to get definite separation of the particles. This can be seen from FIGURE 1 in which the full line X shows a typical relation between the apex discharge and particle size in a conventional cyclone elutriator. Let us say that A represents the bourrdary size of the separation. it will be seen that all particles of size greater than A will be discharged through the apex orifice but fractions of a size less than A will be discharged partly through the apex orifice and partly through the vortex finder. Take the case of particles of size B; in the case shown 50% of these will discharge through the apex and 50% through the vortex finder. In the case of very small particles (size these will be discharged in proportion x percent to the apex orifice and (l00x) percent to the vortex finder. Thus the discharge through the apex orifice contains particles of all sizes and the separation is ill-defined.

With apparatus according to the invention there is no net underflow discharge during the separation and solids entrained in the fluid swirling within the cyclone and the sample container are given a number of passes through the flow pattern of the elutriator dependent upon their size and the length of time for which elutriation is carried out. The lightest particles are discharged quickly through the vortex finder and the remaining fractions are continuously passed through the cyclone and more and more of particles of size less than the separation size are discharged through the vortex finder until ultimately only those particles of the desired separation size and over are left in the cyclone. During the elutriation the following sequence occurs in the elutriator shown in FIG- URE 2:

Depending on suitable selection of the feed opening diameter, vortex finder length and orifice diameter, apex orifice diameter, sample container dimensions, and fluid flow rate, a certain proportion of the swirling fluid moves along the conical side 2 of the cyclone 1 and enters the sample container 7. This continuously stirs the solids in the sample container and returns, carrying solids of all sizes, via a stable swirling flow, to the cyclone. The fluid then moves through a precise path defined by an imaginary cylinder of relatively small diameter, coaxial with the axis of the cyclone and discharges through the vortex finder 6 together with the remaining water (i.e. with the fraction of the water which did not enter the sample container).

Consequently, all particles independent of size, enter the central portion of the well known cyclone flow pattern and due to centrifugal forces tend to settle outwards at rates dependent on their relative settling velocities. As a result, the fastest settling particles leave the flow towards the vortex finder (through the imaginary cylinder) in regions near the apex and are returned to the sample container by the fluid flow. Particles with slower settling rates travel nearer to the overflow orifice in the vortex finder before leaving the same fluid and returning to the fluid flowing towards the sample container.

Particles with slow settling rates do not leave the flows reporting to the vortex finder and are discharged from the apparatus. The whole cycle of operations is repeated continuously for a suitable chosen elutriation time. As the elutriation time increases the resultant separation becomes rnore sharply defined i.e. approaches an idealised separation.

By suitable choice of the tangential feed diameter, vortex finder length and diameter, apex diameter and fluid flow rate, and other cyclone characteristics, the actual size of separation can be adjusted and controlled within wide limits (40 to 8 microns in trials with the present apparatus). In effect all particles finer than a predetermined size are discharged from the system and all particles coarser than that size are retained. This can be seen by reference to FIGURE 1 wherein the broken line Y indicates the relation between apex discharge and particle size for an elutriator of the type shown in FIGURE 2. This curve can be regarded as one of a family of curves of which X represents the special case of a single pass separation in a conventional elutriator and the line through A represents the special case of the ideal separation wherein the elutriation is carried on under ideal conditions for an infinite time to give an infinite number of passes.

A typical sizing test is performed as follows:

The sample container 7 is removed and a weighed sample of solids to be elutriated is inserted together with water (if this is the selected fluid) and a suitable dispersant. The container is almost completely filled.

The metal rod 8 is then pushed down so that the rubber plug 9 seals the opening 10 in the base of the container which, in operation, mates with the apex orifice 4. The container and its contents are agitated to ensure conditions necessary for good dispersion, and the container is then connected to the apex orifice of the cyclone.

Fluid (e.g. water) is introduced into the cyclone through the tangential feed opening 5, at a considerably greater flow rate than the one selected for the actual elutriation period. This ensures that there is no detrimental start up efiect. The metal rod is then pulled upwards, removing the rubber plug from the base of the sample container and placing the container in communication with the interior of the cyclone. Water from the cyclone passes through the sample container as described above. After say, five minutes operation during which all the very fine particles are removed from the elutriator through the vortex finder 6, the flow rate is decreased to a predetermined value dependent on the separation to be achieved and is maintained at that value for the selected elutriation time. The flow rate is then increased to a much higher value and the small discharge valve 11 in the sample container is opened thus discharging all the remaining solids from the cyclone and sample container. These are dried and weighed.

As stated previously, the known cyclone characteristics are chosen to give the desired separation. In order to ensure that adequate distribution of all the sample material through the cyclone flow is achieved the apex orifice size and the size and proportions of the sample container must be chosen so that adequate stirring of the material in the sample container takes place, i.e. care must be taken to ensure that a sufiicient proportion of the flow can enter the sample container in an annular stream, circulate there-through and return through the apex orifice to the cyclone as a vortex stream.

Tests have shown that the method is highly reproducible and gives very precise separations.

The apparatus above described can be used in two particular methods of size analysis to obtain a number of size fractions. In one method a number of individual elutriators are used to separate diflerent'selected size fractions from samples of a test material. The characteristics of flow rate, feed diameter, vortex finder length and diameter and apex orifice diameter etc. are chosen to give the desired separation in each elutriator. For example, the first may be chosen to retain all particles of size 30 microns and greater, the second to retain particle sizes of 25 microns and greater and so on.

In the second method, which is illustrated in FIGUREv 3, a number of similar cyclones 20, 21, 22 are arranged.

. i in series and are designed to separate out size fractions of progressively smaller particles. For example, the first cyclone 20 may be designed to retain particles of size 30 microns and greater, the second 21 to retain particles of size 20 to 30 microns and the third 22 to retain particles of size to microns. In this arrangement the vortex finder discharge from the first elutriator is led to the inlet to the second and that from the second elutriator is led to the inlet to the third. The sample to be elutriated is placed in the sample container of the first elutriator 20 and the process is operated in the manner previously described for the single elutriator. It will be appreciated however that there is no need to provide plugs for the second and subsequent elutriators. After the required elutriation time has elapsed the elutriators are discharged sequentially starting from the last and proceeding to the first and the discharge from each is dried and weighed.

It has been found to be advantageous to eliminate all air from the system before commencing the separation.

It will be appreciated that no discharge of solid particles or of fluid occurs through discharge valves 11 until after the elutriation is completed and that, consequently, the fluid flow rate through each of cyclones is the same at all times. Thus, the whole system only requires one device for controlling fluid flow rate. Each cyclone unit retains particles within the limits of size selected for that unit but passes on particles finer than smallest size limit, and the only particles permitted to leave the whole system continuously are those finer than the smallest size limit of the last unit 22 in the series, which are eliminated with the fluid flowing through the vortex finder of 2-2.

The sample to be elutriated is placed in the sample container of the first elutriator 20 and the process is Operated in the manner previously described for the single elutriator. l p

During operation each of the elutriators behaves in a manner similar to that already described in detail for a single elutriator in that all the particles retained in each elutriator are continuously recycled to the separating zones thereby increasing the precision of separation with increase in time of operation.

It will be appreciated that such an arrangement of cyclone elutriators will behave in a manner diiferent from conventional cyclones arranged in a similar manner, in that conventional cyclones involve the continuous discharge of solids and fluid from the cystem via each apex orifice and, as a consequence, the solids in the discharge through each apex orifice cannot be subjected to repeated recycling to the separating regions within each cyclone. Consequently the apex discharge from each conventional cyclone will contain a wide size range of particles, even including some of the finest particles in the original sam ple.

In a modification of the series arrangement designed for accommodating large samples, the sample material is introduced from outside the first elutriator. The arrangement shown in FIGURE 4 illustrates one of several methods by means of which this can be achieved. It will be appreciated that the method used does not comprise part of the invention but merely permits one form of use of the invention. In a typical operation a relatively large sample, say 100 grams, of the solid material to be elutriated, dispersed in the working fluid is placed in the sample container 23 with valves 24 and 25 so adjusted that the fluid feed cannot enter container 23, but passes by the conduit, parallel to container 23, directly to the first elutriator of the series. The fluid feed is set to a fixed flow rate 1.2 to 2.0 times that to be used for the elutriation operation, all air removed from the system, and sufiicient time allowed for the flow patterns in the whole system to become stable. Valves 24 and 25 are then adjusted so that sufficient of the fluid feed is diverted through container 23, such that the solid sample is con- 6 tinuously displaced from the container and just completely displaced over a fixed period of, say, 10 minutes. During this period the total fluid flow rate is maintained constant and the solid particles are distributed between elutriators 20, 21 and 22, and particles which are too fine to be retained by the last elutriator 22, even under the high flow rate conditions, are eliminated through the vortex finder 6 of elutriator 22. This feeding procedure has been established as ensuring that inaccuracies due to particle to particle interference and start up conditions are reduced to insignificant levels. The fluid flow rate is then reduced to a predetermined elutriation rate for the required time. The flow rate is then increased to the previous high value and the elutriators discharged sequentially as in the previous arrangement. In this case, of course, none of the containers are provided with plugs.

A typical installation may employ elutriators having the following basic dimensions:

For each unit the dimensions of feed inlet diameter and vortex finder diameter were chosen to efiect a predetermined boundary of separation as set out in the following :table under the following operating conditions:

FIuid Water at 20 0.

Flow rate during e1utr-iation2.5 Imperial gallons/min. Flow rate at starting and stopping3.1 Imperial gallons/min.

Elutriation time30 minutes Samp1equartz of specific gravity.2.7

Unit No i 20 21 y 22 Feed diameter (inches); A Vortex finder diameter (inches) Boundary size of separation (microns) 23 15 All air was dispelled from the system before commencing separation.

It can be shown that for given operating conditions and cyclone characteristics the relationship between flow rate (G) and boundary size of separation (d) can be expressed as follows: duG' provided that the flow rate is sufiicient to provide enough energy for cyclone operation and to ensure adequate circulation through the sample container.

Laboratory appraisal tests using solid perspex spheres have shown that, under appropriate conditions similar to those set out above, after an elutriation time of 30 minutes, at least of the particles retained in any one unit lay within the ideal size range.

Cyclone elutriators constructed and operated as above described have the following advantages in an elutriation of relatively fine solid particles. They (a) are very simple in design and construction;

(b) are very easy to control and operate;

(0) utilise centrifugal forces but involve no moving parts;

(d) involve very stable flow patterns, relatively unaffected by room temperatures or draughts;

(e) permit relatively large samples to be used and can produce separate sized fractions with high precision of separation in relatively short times;

( are readily adjusted to give precise separations in the range 40 to 8 microns for material such as quartz (SG 2.7); and

(g) are operated on a repeated pass principle.

It is to be understood that the method and apparatus of this invention are not limited to the particular details which have been described above and that many modifications and adaptations in details of construction and operation may be made without departing from the spirit and the scope of this invention.

I claim:

1. Apparatus for separating a sample of solid particles into two separate sized fractions, comprising a conical cyclone elutriator having a fluid inlet arranged to introduce fluid tangentially into the elutriator, means to introduce all the particles of the sample into the fluid stream of the elutriator, a vortex finder, an underflow orifice at the apex of the cyclone, said cyclone having an inner surface converging to said orifice, means defining a generally cylindrical chamber arranged coaxially with said cyclone, said chamber having a cylindrical wall surface portion with a diameter larger than the orifice and a diverging wall surface portion extending from said orifice to said cylindrical wall surface portion so that said underflow orifice opens directly and without restriction into said chamber whereby the whole of the underflow discharge of the cyclone can be continuously circulated through said chamber and returned to the cyclone, and a discharge valve in said chamber operable to permit discharge of the contents of the elutriator after elutriation has been concluded.

2. Apparatus for separating a sample of solid particles into two separate sized fractions, comprising a conical cyclone elutriator arranged with its apex uppermost and having a fluid inlet arranged to introduce fluid tangentially into the elutriator, an underflow orifice at the apex of the cyclone, a vortex finder at the base of the cyclone, a detachable container for the sample defining a generally cylindrical enclosed chamber arranged coaxially with the cyclone into which said underflow orifice opens, said chamber being provided with a discharge valve and a removable plug to seal the chamber from the interior of the cyclone; withdrawal of the plug permitting the introduction of the particles of the sample to the fluid stream of the elutriator and placing the interior of the cyclone into direct communication with the interior of the chamber, whereby the underflow orifice of the cyclone is enclosed by the chamber and the whole of the underflow discharge of the cyclone can be continuously circulated through the enclosed chamber and returned to the cyclone.

3. The method of separating a sample of solid particles into two sized fractions which are respectively greater than and less than a chosen boundary size of separation,

comprising the steps of introducing all of the particles of the said sample into a cyclone elutriator while fluid is introduced into the latter, such elutriator being modified by enclosing the underflow orifice Wth an enclosed chamber so that there is no net underflow discharge, and said underflow discharge is circulated through the chamber and returned to the cyclone, continuing the introduction of fluid to the elutriator for a predetermined separation period while controlling the flow rate chosen to give the chosen boundary size of separation, whereby all particles of the said greater size fraction are retained in the elutriator and caused repeatedly to traverse the flow pattern thereof and substantially all particles of said lesser size fraction are given suflicient passes through the flow pattern to permit them to be discharged with the vortex finder discharge, and discharging the retained particles after said predetermined separation period.

4. The method according to claim 3; wherein, during an initial period, fluid is introduced to the elutriator at a flow rate greater than said flow rate for the chosen boundary size of separation, and then the flow rate is reduced to said flow rate for separation and is maintained at said flow rate for separation for said predetermined separation period whereupon the flow rate is again increased during a final period; and wherein the sample is introduced to the elutriator during said initial period of increased flow rate and the retained particles are discharged during said final period of increased flow rate.

References Cited by the Examiner UNITED STATES PATENTS 2,550,341 4/51 Fontein 209211 2,709,397 5/55 Banning 209211 2,754,968 7/56 Vegter 209-211 2,835,387 5/58 Fontein 209211 2,958,420 11/60 Hurley 209--2l1 3,016,962 1/ 62 Lummus 209-211 XR FOREIGN PATENTS 74,447 4/ 5 4 Netherlands. 817,342 7/ 59 Great Britain.

HARRY B. THORNTON, Primary Examiner.

ROBERT A. OLEARY, HERBERT L. MARTIN,

Examiners. 

1. APPARATUS FOR SEPARATING A SAMPLE OF SOLID PARTICLES INTO TWO SEPARATE SIZED FRACTIONS, COMPRISING A CONICAL CYCLONE ELUTRIATOR HAVING A FLUID INLET ARRANGED TO INTRODUCE FLUID TANGENTIALLY INTO THE ELUTRIATOR, MEANS TO INTRODUCE ALL THE PARTICLES OF THE SAMPLE INTO THE FLUID STREAM OF THE ELUTRIATOR, A VORTEX FINDER, AN UNDERFLOW ORIFICE AT THE APEX OF THE CYCLONE, SAID CYCLONE HAVING AN INNER SURFACE CONVERGING TO SAID ORIFICE, MEANS DEFINING A GENERALLY CYLINDRICAL CHAMBER ARRANGED COAXIALLY WITH SAID CYCLONE, SAID CHAMBER HAVING A CYLINDRICAL WALL SURFACE PORTION WITH A DIAMETER LARGER THAN THE ORIFICE TO SAID CYLINDRICAL WALL SURFACE PORTION SO THAT SAID UNDERFLOW ORIFICE OPENS DIRECTLY AND WITHOUT RESTRICTION INTO SAID CHAMBER WHEREBY THE WHOLE OF THE UNDERFLOW DISCHARGE OF THE CYCLONE CAN BE CONTINUOUSLY CIRCULATED THROUGH SAID CHAMBER AND RETURNED TO THE CYCLONE, AND A DISCHARGE VALVE IN SAID CHAMBER OPERABLE TO PERMIT DISCHARGE OF THE CONTENTS OF THE ELUTRIATOR AFTER ELUTRIATION HAS BEEN CONCLUDED. 